1. Field of the Invention
The invention generally relates to an apparatus and method for the manufacture of high quality communication cables of the type including a single set or a plurality of sets of twisted wires.
2. Description of the Prior Art
Communication cables of the type that include a plurality of twisted wires are manufactured in either one stage or in two stages.
In the case where cables are manufactured in two stages, the twisted wires are first prepared by twisting the wires together by means of so-called twinning or pairing machines. Twisted wires are then made up into communications cables by means of for example, stationary take-ups, rotating take-ups (also called drum twisting machines) or other types of rotating equipment.
One form of equipment conventionally used for twisting two, three or four wires is the double twist machine. The resulting twisted elements are called pairs, triads or quads.
This equipment includes a bobbin cradle around which is arranged a rotatable frame or bow which is driven to turn around the cradle. Wires to be twisted may be supplied from bobbins on the bobbin cradle inside the twinning cage and taken up on a take-up reel outside the twinning cage. The aforementioned arrangement is referred to as an "inside-out" machine. The wires to be twisted may also be supplied from outside the twisting cage and taken up on a bobbin arranged within the bobbin cradle. The latter configuration is sometimes referred to as an "outside-in" machine.
Outside-in machines are generally preferred in individual twisting machines since the wire may be supplied from storage facilities of simple construction and greater capacity. In this case, the bobbin cradle within the twisting cage is also required to hold only a single bobbin. The outside-in machine is also readily adaptable to use with a greater number of wires.
If communication cables are made in one stage, the apparatus generally employs a plurality of twisting machines, or heads of the "inside-out" type.
The twisted elements so manufactured are directed to any type of take-up (e.g., stationary or rotating take-ups, single or double twist machines, capstan or extrusion lines) for laying up twisted wires to form a communication cable. This is done in one operation.
The plurality of double twist twisting machines can be arranged horizontally or vertically, depending on the preferred plant layout.
One typical example of such an installation is disclosed in U.S. Pat. No. 5,400,579 assigned to the assignee of the subject application.
It is well-known in the art that the lay obtained with double twist actions is not perfectly regular and, if longer lays are used, some irregularity in the position of the cores in the twisted elements have to be accepted in order to achieve higher speed of manufacture. These irregularities in the lays do not cause problems in communication cables such as low frequency telephone cables used in standard telephone applications since the perfect constancy of the lays and in the relative position of the individual wires in each element (pair, triad or quad) are not that critical.
With the advent of high speed data transmission, especially for computer use and other applications, the frequencies required are much higher and therefore standard pairs, triads or quads acceptable in telephone networks cannot be used in such high frequency applications.
It is well known, for example, that the characteristic impedance of an n-wire cable is a function not only of the diameters of the individual conductors but also a function of the spacing or distances between the conductors. Matched impedances are critical at high frequencies to optimize power transfer, reduce line reflections which cause deterioration of signal integrity and optimize the useful frequency band width for which the cable can be used.
It has been proven that, for example, the characteristic impedance of pairs can change drastically at different frequencies around its theoretical average. Cables utilizing high quality pairs have been produced for use in communication local area networks (LANs) with a maximum useful frequency of 100 MHz. This, in the industry, is called a Level or Category 5 cable. The specification for these cables requires, for example, that the nominal characteristic impedance of 100 Ohms can only vary between 85 and 115 Ohms from 0 to 100 MHz.
The industry is already requiring twisted elements, especially pairs, that will maintain their electrical characteristics up to and above 600 MHz. This is normally called an "enhanced" Category 5 or Category 6 communication cable.
In order to produce pairs, triads or quads that can operate satisfactorily at these frequencies, it is necessary to produce a cable in which the individual elements or wires of each pair, triad or quad ideally be maintained substantially in the same desired positions relative to each other so that the electrical characteristics of the pair, triad or quad vary within specified ranges along the length of the cable.
One acceptable way of achieving this has been to shorten the lays of the elements in order to manufacture an element that is mechanically more stable. This approach has, however, reduced the productivity of the equipment since there are physical limitations on the rotational speeds of the bows used in double twist machines.
Another approach for maintaining the mechanical integrity of assembled cable is disclosed in U.S. Pat. No. 5,622,039, assigned to the assignee of the subject application, which uses a group twinner in which each wire twister includes an internal tape dispenser for taping the wire pairs before assembly of the cable.
A still further approach is disclosed in U.S. Pat. No. 5,606,151 for a twisted parallel cable intended for high frequency transmission use that includes a plurality of insulated conductors that are twisted to form a pair. The pairs of adjoining insulated conductors are encased within a thermoplastic, fluorocopolymer or rubber type material.
However, "physically" maintaining the relative positions of the individual wires along the length of the cable is not sufficient as the frequency of operation is pushed higher and higher, where factors not visible at lower frequencies become important considerations. Because impedance is a function of the spacing between the conductors, variations in the eccentricities of the conductors within their insulating sheaths also impact on the spacings between the conductors. In most cases, the conductors are never precisely concentric in relation to their exterior insulations, most conductors being within the range of 88% to 95% concentricity. This means, however, that there is more insulation on one side of a conductor than on the other, thus creating physical bumps or high spots, on one side, and low points, on the other. Because two forces are created when two wires are twisted, one that twists the wires and the other that is directed toward the center, a twisted pair will typically arrange the individual wires to be in abutment at the thinnest portions of the insulation. These regions of reduced interconductor spacing create corresponding regions of lower impedance. As suggested, at lower frequencies such low spots caused by variations in eccentricity are not consequential. However, as the wavelength of the signal frequencies approach the distances between such low spots this problem becomes more significant. As data transfer is pushed from 100 megabits/sec. to 600 megabits/sec any deviations that effect the electrical properties of the twisted conductors are as significant as the factors that maintain the mechanical integrity of the cable.
It has been observed that by torsioning the individual wires about their own neutral axes prior to twinning the high and low spots on the twinned wires are made to shift along the cable, this having the effect of averaging or smoothing out impedance variations and having beneficial results on the overall cable, reducing structural return losses (SRLs) as well as the impedance fluctuations over the anticipated frequency ranges. See, for example, FIGS. 1 and 2 which show the impedance and SRL characteristics of a cable made with a planetary machine, which provides full or 100% backtwist on the individual wires prior to twinning, and FIGS. 3 and 4, showing the impedance and SRL characteristics of a cable made on a rigid machine with a zero backtwist. These differences can best be explained by referenced to FIGS. 5 and 6.
In FIG. 5 a pair of insulated wires 10, 12 are shown in abutment or in contact with each other at a point or, more accurately, a helical line 14. For purposes of simplicity the conductor 10a of the wire 10 is shown to be perfectly concentric within the insulating sheath 10b (concentricity=100% or eccentricity=0). The conductor 12a of the wire 12, however, is eccentric in relation to the insulator 12b, the extent of eccentricity being defined as e=(t.sub.1 /t.sub.2 .times.100)%. As a result, the interconductor spacing S is less than the diameter of the wires, as it would be if both conductors were perfectly concentric. The wire 10 is labeled with a triangular marker 10c while the wire 12 is labeled with a dot marker 12c for establishing reference points of angular orientation of these wires about their own axes. The wire pair P in FIG. 6a develops a helix having a length I which is a function Do equal to the diameter described by the processed members, the amount of torsion being a function of the nature of the machine performing the twinning. For a rigid frame machine the torsion is: EQU Torsion=360.degree. L/[(.pi.D.sub.0).sup.2 +L.sup.2 ].sup.1/2 (1).
For a planetary machine the torsion is: EQU Torsion=360.degree. L/[(.pi.D.sub.0).sup.2 +L.sup.2 ].sup.1/2 -360.degree.(2).
It is evident from equations 1 and 2 that for very small diameter wires the torsion for a rigid-frame machine is about 360.degree. over one lay length (FIG. 6c), while that torsion is about 0.degree. for a planetary machine (FIG. 6d). In FIGS. 6c and 6d, each of the wires are illustrated at 0.degree., 90.degree., 180.degree., 270.degree. and 360.degree. intervals or positions along the helical twist, showing both how the individual wires have been torsioned about their axes and about themselves. With the rigid machine, the wires in FIG. 6c rotate equally about each other as well as about their individual axes so that the wires continue to contact along the same line 14. However, in FIG. 6d, for the planetary machine, the wires twist about themselves although they maintain their individual angular orientations fixed throughout the helix. For this reason the markers 10c, 12c remain fixed at the 12:00 o'clock positions along the helix while they are twisted about each other when made on a planetary machine.
From FIGS. 1 and 2 it is clear that the torsioning or rotating of the wires 10, 12 about their individual axes with a planetary machine (FIG. 1) improves the impedance characteristics of the twisted pair, reducing the impedance variation to approximately 10 Ohms over the frequency range of 0-100 Mhz, while the wires formed by a rigid machine (FIG. 3) provide much greater swings and exceeds UL specifications at a number of frequencies by dropping below 85 Ohms or exceeding 115 Ohms. While this suggests that high frequency pairs for Category 5 and 6 cables should be made on planetary machines, such machines are not the machines of choice for these applications, and rigid machines are used almost exclusively because of their better productivity for stranding, pairing, etc. However, rigid machines that pre-twist the individual wires prior to twinning, etc., have not been used with group twinners to efficiently produce high frequency cables that have enhanced high frequency products.